1. Field of the invention.
The present invention relates to an electrophotographic machine, and, more particularly, to a method and apparatus for minimizing visual artifacts resulting from laser scan process direction position errors.
2. Description of the related art.
In an in-line color electrophotographic imaging process, latent images are formed on a plurality of photosensitive drums, which are in turn developed using a predetermined color of toner. The developed images are then transferred to a sheet of media (such as paper) which travels past the drums. The image in each color is created one line at a time, and the lines are oriented at right angles to the direction of travel of the sheet of media. The individually-generated images combine to form a full-color image. In a typical multi-color laser printer, for example, the sheet of media passes through four color developing stations in series, with the colors being black, magenta, cyan and yellow.
It is recognized that in order for the multi-color laser printer to print accurately, the laser beams for all four colors must be in alignment, both in the scan direction (across the page) and the process direction (feed direction of the print medium). However, providing proper alignment of even a single laser printhead in relation to the sheet of media in the process direction can be difficult. This problem is compounded with the addition each printhead, since the plurality of printheads must be in registration so that the individual images generated by each printhead can be superimposed correctly when combined. During printer assembly an attempt is made to optically align the laser printheads both individually and collectively, but the ability to provide precise alignment is limited by several factors, including component tolerances. In addition, it is possible for a precisely aligned printing system to drift out of alignment due to component aging and/or the influences of printing environment factors, such as the internal temperature changes of the printer that cause distortion of the laser beam optical scan path.
What is needed in the art is a method and apparatus for minimizing visual artifacts resulting from laser scan process direction position errors, such as printhead skew and laser beam optical scan path distortion.
The present invention provides method and apparatus for minimizing visual artifacts resulting from laser scan process direction position errors, such as printhead skew and laser beam optical scan path distortion.
One aspect of the invention relates to an electrophotographic machine having a process direction. The electrophotographic machine includes a photoconductive member having an image forming surface having defined thereon a plurality of pixel locations which form a pixel grid, wherein two adjacent pixels in the process direction are separated by a pixel spacing distance. The electrophotographic machine also includes a printhead unit having a first laser beam generator and a second laser beam generator. The first laser beam generator generates a first laser beam which impinges the surface of the photoconductive member at a first location and the second laser beam generator generates a second laser beam which impinges the surface of the photoconductive member at a second location. The second location is spaced apart from the first location in the process direction by a separation distance equal to an integer multiple of the pixel spacing distance plus a distance less than the pixel spacing distance.
In one method for minimizing visual artifacts resulting from laser scan process direction position errors, error correction is provided by selectively operating the first laser beam generator and the second laser beam generator during a formation of a scan line on the surface of the photoconductive member.
An advantage of the present invention is that the scan path of the laser beams of a multicolor electrophotographic machine can be corrected for printhead skew and optical scan path distortion, both during and after an initial printhead alignment.
Another advantage of the invention is the ability to synthesize the position of spot formation on the photoconductive drum in the process direction.